1. Field of the Invention
The present invention relates generally to the cleaning of a semiconductor substrate and, more particularly, to a method and apparatus for utilizing acoustic energy in combination with selective cleaning media to clean a semiconductor substrate following fabrication processes.
2. Description of the Related Art
In the semiconductor chip fabrication process, it is well-known that there is a need to clean and dry a substrate where a fabrication operation has been performed leaving unwanted residues on the surface of the substrate. Examples of such fabrication operations include plasma etching (e.g., via etch or trench etch for copper dual damascene applications) and chemical mechanical polishing (CMP). Various cleaning processes have been used in removing the unwanted residues from the surface of the substrate after each fabrication operation. Some of the cleaning processes that are used in removing the unwanted residues from the surface of the substrate involve propagation of acoustic energy through a liquid medium that substantially covers the surface of the substrate and using the acoustic energy to remove particles from the surface of the substrate.
Typical liquid medium that have been used during acoustic energy cleaning processes include deionized water (DIW) or any one or more of several substrate cleaning chemicals and combinations thereof such as dilute ammonium hydroxide/hydrogen peroxide solution in DIW. The propagation of acoustic energy through the liquid medium enables cleaning chiefly through cavitation, microstreaming, and chemical reaction enhancement when chemicals are used as the liquid medium. Cavitation is the rapid forming and collapsing of microscopic bubbles from dissolved gases in the liquid medium under the action of sonic agitation. Upon collapse, the bubbles release energy. The energy from the collapsed bubbles assists in particle removal through breakage of the various adhesion forces that allow the particle to adhere to the substrate. Acoustic microstreaming is the fluid motion induced by the velocity gradient from propagation of an acoustic wave through the liquid medium under megasonic vibration. The acoustic energy provides the activation energy to facilitate the chemical reactions within the liquid medium.
As the critical dimensions of features formed on the surface of the substrate continue to decrease, the number of submicron particles with dimensions that are comparable to the critical dimensions of the features increases. Due to the critical dimensions of the submicron particles, cleaning techniques using low-frequency ultrasonic energy are no longer capable of effectively removing the submicron particles from around the features. As a result, the reliability and yield of the resulting devices from the fabrication operation using ultrasonic cleaning are substantially reduced.
To overcome the ineffectiveness of the ultrasonic cleaning, megasonic energy was widely used in semiconductor manufacturing operations to clean a batch of wafers or a single wafer. The main particle removal mechanisms by megasonic cleaning are through cavitation and acoustic microstreaming. A megasonic transducer is used to create sonic pressure waves in the liquid medium within a cleaning tank in which the substrate or batch of substrates are immersed. Unwanted particulates are acted on through cavitation and microstreaming. Although the acoustic energy used in the megasonic cleaning effectively removes the unwanted particles from the surface of the substrate, the amount of acoustic energy used for providing cavitation may result in damage to the features around which the submicron particle contaminants are deposited. The damage to the features makes the associated device inoperable thereby reducing the overall yield and reliability of the devices obtained from the substrate.
It would be advantageous to find an approach for applying acoustic energy to effectively remove submicron particulate contaminants from the surface of substrates without damaging the features formed on the surface of the substrate.
It is in this context that embodiments of the invention arise.